1. Field of the Invention
This invention relates generally to plasma processing systems, and more particularly to radio frequency (RF) power generation and distribution in such systems.
2. Description of the Background Art
Plasma processing systems are employed in the fabrication of semiconductor devices. In a plasma processing system, such as a plasma enhanced chemical vapor deposition (PECVD) system, radio frequency (RF) power is supplied to the system's chamber to maintain a plasma therein. A semiconductor substrate in the chamber is exposed to the plasma and process gases, thereby performing a fabrication process on the substrate. In a PECVD system, for example, the fabrication process may be deposition of a thin film of dielectric material on the substrate.
FIG. 1 shows a schematic diagram of a typical RF delivery configuration. In the configuration of FIG. 1, an RF generator 101 is coupled to one or more electrodes in a plasma processing chamber 105. RF generator 101 may be a so-called “high-frequency” RF generator operating at a frequency of about 13.56 MHz and having an output impedance of about 50 Ω (Ohms). RF generator 101 is coupled to plasma processing chamber 105 via a 50 Ω coaxial cable 102. Coaxial cable 102 typically has a fixed length specifically selected for the characteristics required by plasma processing chamber 105.
To prevent power loss due to signal reflection, impedance matching network 103 performs an impedance transformation to match the load impedance presented by plasma processing chamber 105 to the characteristic impedance of cable 102. Impedance matching network 103 may be an automatic impedance matching network or a fixed impedance matching network. An automatic impedance matching network, which is also referred to as an “auto-match,” is so named because it employs tuning components that are automatically adjusted in response to the impedance presented to it. A fixed impedance matching network, which is also referred to as a “fixed-match,” is so named because it employs components that are not adjusted during normal operation. That is, the components of a fixed-match have preset values. A matching network 103 may be connected to an electrode in plasma processing chamber 105 using a connection scheme 104, which may include combiners, splitters, direct connections using electrically conductive elements, or combinations thereof.
The load impedance presented by a plasma processing chamber can have relatively high phase angles, which can lead to high circulating currents and RF delivery stability issues. Matching network 103 thus requires components that can handle high circulating currents, thereby increasing the cost of matching network 103. The high load currents also raise reliability issues by degrading the components of matching network 103, lower the efficiency of matching network 103 because of I2R power losses, and prevent accurate voltage-current probe readings between matching network 103 and the chamber.
In the example of FIG. 1, matching network 103 is customized to the load impedance presented by chamber 105. Thus, a manufacturer offering different models or types of plasma processing chambers may have to design, manufacture, and stock different matching networks 103. This not only increases the overall cost of the plasma processing system, but also raises logistical issues.